This invention relates to swept frequency signal generators and more particularly to tuneable filters for use in microwave swept frequency signal generators.
Swept frequency signal generators are used extensively for testing radio frequency and microwave communications equipment. Because of the broad range of signal frequencies used in modern communication equipment it is desirable to have test instrumentation that will cover a broad range of frequencies. Prior art swept frequency signal generators typically cover a broad range of frequencies by sweeping over several contiguous bands. Such a prior art device is described in an article entitled "A High-Performance 2-to-18-GHz Sweeper" by Paul R. Hernday and Carl J. Enlow in the Hewlett-Packard Journal; March 1975, pages 1-14. The lowest band is usually the fundamental frequency of the sweeping oscillator in the signal generator, for example 2 to 6 GHz. This frequency is multiplied, for example with a step recovery diode multiplier, to produce the higher frequency bands, e.g. 6 to 12 GHz, 12 to 18 GHz, etc. Since a diode multiplier produces many different harmonics of the fundamental at the same time, it is necessary to put a filter on the output of the multiplier in order to select the desired signal frequency. The most suitable type of filter for a multiband swept frequency signal generator is a tracking band pass filter using a yttrium-iron-garnet sphere as the tuneable resonant element, usually called a YIG filter.
One of the disadvantages of the type of swept frequency signal generator just described is that it is sometimes desirable to have a starting frequency lower than 2 GHz, sometimes as low as a few megahertz. In the prior art the provision of such low frequencies required both a separate low frequency band generator to supply the swept low frequency signal and a cumbersome and lossy electromechanical broad band switch to switch between the high frequency output of the YIG filter and the output of the low frequency band generator. Because such electromechanical switches are inherently slow compared with the sweep rates typically used for testing microwave communication circuits, it was difficult have a signal that was continuously swept from a few megahertz to 18 GHz or more. In addition, even if the sweep rate were low enough to accommodate the switching speed of such switches they would often wear out under the repetitive use usually given a swept frequency signal generator.